Automatic image registration of scans for image-guided surgery

ABSTRACT

Provided is a method for determining a position of an imaged anatomical body part of a patient. The method includes acquiring patient image data describing a digital image of at least part of a reference device and the anatomical body part, acquiring reference device model data describing a model of at least one of at least one internal surface or at least one external surface of the reference device, determining, —based on the patient image data and the reference device model data, reference device image position data describing a relative position between the reference device and the anatomical body part, acquiring reference device tracking data describing a position of the reference device in the tracking reference system, and determining, based on the reference device image position data and the reference device tracking data, body part tracking data describing a position of the anatomical body part in the tracking reference system.

RELATED APPLICATION DATA

This application is a National Phase Application of InternationalApplication No. PCT/EP2016/068764 filed Aug. 5, 2016 and published inthe English language.

The present invention relates to a computer-implemented method fordetermining a position of an imaged anatomical body part of a patient ina tracking reference system, a corresponding computer program, anon-transitory program storage medium storing such a program and acomputer for executing the program, as well as a system for supportingdetermining a transformation defining a geometric relationship between aposition of an anatomical body part of a patient in an image referencesystem and a position of the anatomical body part in the trackingreference system, the system comprising an electronic data storagedevice and the aforementioned computer. Furthermore, the presentinvention relates to a reference device for use in a navigated medicalprocedure and use of the reference device.

TECHNICAL BACKGROUND

Currently, automatic registration of an anatomical body part to atracking reference system is performed by attaching markers to ascanner, calibrating image space relative to the scanner-markers using acalibration phantom and performing registration for use in surgery byregistering physical space to the scanner-markers and adding thetransformation from there into image space. One approach includesautomatic registration by embedding fiducials into a reference deviceand scanning the reference device. Another approach includes automaticregistration by attaching fiducials to the skin of the patient. A thirdapproach includes automatic registration by holding a registrationdevice with embedded fiducials into the scan area and registering it tothe dynamic reference. A fourth approach includes automatic registrationby attaching a registration device to the patient and attaching a robotto the registration device and to the patient.

All known solutions require an array of fiducials, e.g. image-visibleCT- or MR-markers, which are typically metal or glass spheres, rods orother components made of a material with different density than thesurrounding material. This applies for registration in x-ray basedmodalities (CT, Cone-beam-CT, O-Arm, 3D C-arm etc.). For MR, chambersfilled with MR-visible liquids are used. These fiducials are typicallyembedded into a registration device with high accuracy which requirescertain manufacturing and assembly efforts. The fiducials may causeartefacts and therefore deteriorate the image quality of the 3D scan.Furthermore, not only the markers but the entire registration device isvisible in the scan and distracts the user from the anatomicalinformation.

The present invention is designed to achieve a more reliable method forestablishing a positional mapping (a registration) between an imagereference system and a tracking reference system used by a navigationsystem.

The present invention can be used in connection with a system forimage-guided surgery such as Spine&Trauma 3D, a product of Brainlab AG.

Aspects of the present invention, examples and exemplary steps and theirembodiments are disclosed in the following. Different exemplary featuresof the invention can be combined in accordance with the inventionwherever technically expedient and feasible.

Exemplary Short Description of the Present Invention

In the following, a short description of the specific features of thepresent invention is given which shall not be understood to limit theinvention only to the features or a combination of the featuresdescribed in this section.

The disclosed method encompasses scanning an anatomical body part andfirst a reference device so that the scan image shows both. A geometricmodel of the reference device is known, so that the position of theimage representation of the reference device in the scan image can bedetermined by applying a surface match or image fusion algorithm. Thereference device is provided with optical markers in a predeterminedspatial relationship to those parts of the reference device which arevisible in the scanned image. The marker devices are optically trackedso that their position in a tracking reference system is known. Based onthe knowledge of the relative position between the markers and theimage-visible part of the reference device, a positional transformationbetween positions in the scan image and positions in the trackingreference system is determined so that, based on the scan imagedescribing the relative position between the image-visible part of thereference device, the position of the anatomical body part in thetracking reference system can be determined. In one example, the methodencompasses positioning a second reference device near to theaforementioned (first) reference device. The second reference device isalso provided with optical markers so that a relative position betweenthe markers of the first reference device and the markers of the secondreference device can be determined by a navigation system suitable foroptical tracking. The second reference device may be fixed to thepatient so that the first reference device can be removed from the setuponce the relative position between the markers of the first referencedevice and the markers of the second reference device has beenestablished. The position of the anatomical body part in the trackingreference system can then be determined by additionally considering therelative position between the markers of the first reference device andthe markers of the second reference device.

General Description of the Present Invention

In this section, a description of the general features of the presentinvention is given for example by referring to possible embodiments ofthe invention.

In general, the invention reaches the aforementioned object byproviding, in a first aspect, a computer-implemented medical dataprocessing method for determining a position of an imaged anatomicalbody part of a patient in a tracking reference system. The anatomicalbody part in one example is at least a part of the patient's spine suchas at least a part of a vertebra, but can in other examples be anyanatomical body part. The anatomical body part has been imaged byapplying a medical imaging modality to the anatomical body part. Forexample, a digital image has been generated based on the result of theimaging. Imaging the anatomical body part in one general example casetakes place before the disclosed method is executed, but may in anotherexample case be part of executing the disclosed method. Likewise,generating the digital image imaging the anatomical body part in onegeneral example case takes place before the disclosed method isexecuted, but may in another example case be part of executing thedisclosed method. The tracking reference system is for exampleassociated with a navigation system usable to conduct an envisagednavigated medical procedure, for example at least part of the navigationsystem rests in the tracking reference system. Specifically, theanatomical body part is not necessarily defined to rest in the trackingreference system—rather, any motion of the anatomical body part can bedescribed by positional differences between positions of the anatomicalbody part which are defined in the tracking reference system. The methodcomprises executing, on at least one processor of at least one computer(for example at least one computer being part of the navigation system),the following exemplary steps which are executed by the at least oneprocessor.

In a (for example first) exemplary step, patient image data is acquiredwhich describes (for example defines and/or represents) a digital imageof at least part of a reference device and the anatomical body part. Thereference device is a non-anatomical structure and is described infurther detail below. The reference device is also called firstreference device within this disclosure. In one approach for generatingthe patient image data, both the anatomical body part and the referencedevice have been imaged simultaneously by applying a medical imagingmodality to the anatomical body part and the reference device. Forexample, a digital image has been generated based on the result of theimaging. Imaging the anatomical body part and the reference device inone general example case takes place before the disclosed method isexecuted, but may in another example case be part of executing thedisclosed method. Likewise, generating the digital image in one generalexample case takes place before the disclosed method is executed, butmay in another example case be part of executing the disclosed method.The patient image data therefore in one general example is generatedbefore execution of the disclosed method ensues and is then used as aninput to the disclosed method. In another example case, generating thepatient image data may be part of executing the disclosed method. Thepatient image data is in one example three-dimensional data (e.g.tomographic image data) and may have been generated by applying at leastone of computed x-ray tomography (CT), cone beam CT, magnetic resonanceimaging (MR), sonography (ultrasound imaging), positron emissiontomography (PET), single-photon emission computed tomography (SPECT) orelectron emission tomography (EIT). In another approach for generatingthe patient image data, the digital image may have been synthesized, forexample by generating a synthesized CT or MR from a digital image of theanatomical body part (generated beforehand by applying at least one ofthe aforementioned imaging modalities) and known constructional data(for example a grid model) describing the physical structure of thereference device. In another example, the patient image data may betwo-dimensional data (i.e. defined in exactly two positionaldimensions), for example it may be embodied by a digital image generatedusing a digital imaging device (a camera) operating in the visiblewavelength range.

In a further (for example second) exemplary step, reference device modeldata is acquired which describes (for example defines and/or represents)a model of at least one of at least one internal surface (for example,plane) or at least one external surface of the reference device. Aninternal surface of the reference device is defined to be a surfacelying in the interior of the reference device which represents forexample a cut such as a planar (slice) or curved cut through thereference device. In other words, the model of an internal surface is amodel of the image appearance (at least one of colour values or innerstructure (geometry)) of the reference device on that surface. If aplurality of (specifically, adjacent) internal surfaces are contained inthe model, that plurality describes at least a part of the volume of thereference device represented by that set of internal surfaces. A modelof at least one external surface includes a model of the imageappearance (i.e. at least one of colour values or geometry) of thereference device on a planar or curved exterior (outer, i.e. tangible)surface of the reference device. The reference device model data in oneexample is three-dimensional data, but in another example betwo-dimensional (i.e. defined in exactly two positional dimensions). Itis envisaged that the patient image data and the reference device modeldata have the same dimensionality, i.e. are defined in the same numberof positional dimensions. Generally, the patient image data and thedevice model data are comparable to one another (so that they can bemapped onto one another), i.e. at least the relation between theirresolutions and reference systems used to describe positions in bothdata sets is known, or the resolution or reference systems are at leastsubstantially identical.

The model defines in one example an image appearance of the referencedevice, i.e. an image representation of the reference device. Forexample, the model describes a combination of colour values (such asmulticolour or greyscale values) defining the appearance of thereference device in a digital image. In one example, the model has beengenerated based on digital model image data describing at least oneimage of the reference device. The digital model image data may havebeen generated applying a tomographic imaging modality to at least partof the reference device. The digital model image data in one examplehence is tomographic image data.

In another example, the reference device model data (specifically, themodel) describes a three-dimensional structure of the reference device,for example at least one of an internal or a surface structure of thereference device, which is described for example by a geometrical grid.Corresponding information may be available e.g. from predetermined (i.e.already known) construction data for constructing the reference deviceusing computer-aided design.

Generating the reference device model data in one general example casetakes place before the disclosed method is executed, but may in anotherexample case be part of executing the disclosed method. In a specificexample, the digital model image data may have been generated bycombining (for example, by statistically analysing such as averaging) aplurality of images (e.g. tomographic images or two-dimensional, e.g.optical, images) of the same reference device or different referencedevices (for example, at least one image or a plurality of images ofeach reference device), wherein the different reference devices are ofthe same type (i.e. have the same design). The model may therefore be astatistical image-based model of the reference device.

Thus, the reference device model data may therefore be a volume or outersurface model representing a reference for the image appearance and/orat least one of the external or internal structure (specifically,geometry) of the reference device which may serve as a reference whichmay be compared to the image representation of the reference device inthe patient image data. In this sense, the reference device model datamay function as an atlas for the reference device.

In a further (for example third) exemplary step, reference device imageposition data is determined which describes (for example, defines orrepresents) a relative position between the reference device and theanatomical body part. The reference device image position data isdetermined based on the patient image data and the reference devicemodel data. The relative position between the reference device and theanatomical body part is in one example defined in an image referencesystem in which positions in the digital image described by the patientimage data are defined. The image reference system can be used forplanning an envisaged medical procedure such as radiotherapy or surgery(e.g. spine surgery). The image reference system is predetermined, e.g.known from the geometry of an imaging device used to generate thepatient image data. This serves to determine the position of the imagerepresentation of the reference device in the digital image described bythe patient image data. The reference device image position data isdetermined by comparing the model of the reference device to the digitalimage appearance of the reference device in the digital image describedby the patient image data, specifically by searching the digital imagedescribed by the patient image data for the image features having atleast a predetermined degree of similarity to the image appearance ofgeometry of the reference device described by the model. This can bedone for example by matching the model of the reference device with thedigital image described by the patient image data, for example byapplying an image fusion or surface matching algorithm to the patientimage data and the reference device model data.

In a further (for example fourth) exemplary step, reference devicetracking data is acquired which describes (for example, defines orrepresents) a position of the reference device in the tracking referencesystem.

The reference device tracking data may in a first example be generatedon the basis of electronic signals generated by tracking a (i.e. atleast one marker device) attached to or integrally included in thereference device using a tracking system included in the navigationsystem. Usually, four marker devices are used, which may also be done inthe present case, even though a minimum of only three marker devices isrequired. Using up to six marker devices is usual and may also be donein this case. The marker device may be formed by the reference deviceitself (e.g. by a base part of the reference device) or comprise atleast one marker (for example, a plurality of markers). The spatialrelationship (i.e. at least one of position or orientation) between suchat least one marker and the reference device is predetermined (i.e.known) and may be used as input to the disclosed method. For example,the marker device may have a plurality of pieces of marker foil materialor marker spheres attached to it; the pieces of foil or the markerspheres are in one example reflective (e.g. retroreflective) for atleast one infrared or visible light (such markers are called opticalmarkers within this disclosure) so that, if they are illuminated withlight of the respective wavelength, the reflections can be received by adetection device such as a stereo camera (which is sensitive in theapplicable wavelength range) and transformed into electronic signals,which in turn are then converted into digital data (e.g. using acommonly known analogue-to-digital converter) to generate the referencedevice tracking data. Alternatively, the marker device may comprise atleast one resonant circuit (for example a plurality of resonantcircuits) which are subjected to electromagnetic radiation (for examplein the wavelength range from 350 Hz to 500 kHz) suitable to generateresonance in the circuits, and the resonance signal is detected by adetection device comprising e.g. an antenna array. Such a marker deviceis also called an electromagnetic marker device. The detected resonancessignals are transformed into electronic signals which are then convertedinto digital data (e.g. using a commonly known analogue-to-digitalconverter) to generate the reference device tracking data. In a furtheralternative example, the marker device may comprise electromagneticallyactive markers which are electrically powered to emit light (forexample, from an emission unit such as a light emitting diode) in theinfrared or visible wavelength range, the light then being detected by adetection device such as a stereo camera (which is sensitive in theapplicable wavelength range) and transformed into electronic signals,which in turn are then converted into digital data (e.g. using acommonly known analogue-to-digital converter) to generate the referencedevice tracking data. The relative position between the marker device(marker sphere) and the part of the reference device which is visible inthe digital image described by the patient image data is generallypredetermined (for example, at least one of known or fixed) anddescribed by for example the reference device model data.

In a second example, the reference device tracking data is determined byanalysis of the reference device model data which contains informationon the geometry of the reference device (including the marker device)and therefore on the relative position between a base part of thereference device which is identifiable in the digital image described bythe patient image data and the position of the marker. That relativeposition already allows for establishing a transformation between thetracking reference system and the image reference system.

In a further (for example fifth) exemplary step, body part tracking datais determined which describes (for examples, defines or represents) aposition of the anatomical body part in the tracking reference system.The body part tracking data is determined based on the reference deviceimage position data and the reference device tracking data. Because thereference device image position data is determined based on the patientimage data, the body part tracking data is at least indirectly based onalso the patient image data. The body part tracking data is determinedbased on (specifically, by) determining a transformation between theposition of the reference device in the image reference system and theposition of the reference device in the tracking reference system.Within the meaning of this disclosure, a transformation is understood tobe a linear mapping which may be represented by a matrix and/or matrixmultiplication. Such a mapping may be determined by applying basicconsiderations from linear algebra to the position of the referencedevice in the image reference system and the position of the referencedevice in the tracking reference system. Since the relative positionbetween the anatomical body part and the relative to the referencedevice is defined in the image reference system and is known from thereference device image position data, and the position of the referencedevice in the tracking reference system is known from the referencedevice tracking data, a transformation between the tracking referencesystem and the image reference system (for example, a transformation ofbases between the two reference systems) can be established and used formapping the position of the anatomical body part in the image system tothe position of the anatomical body part in the tracking referencesystem.

In a general case, the relative position between the reference deviceand the anatomical body part should be unchanged between the point intime at which the patient image data is generated and the point in timeat which the reference device tracking data is determined. If this isnot the case, a further (second) reference device may be used which hasthe known (for example unchanged) position relative to the anatomicalbody part and can be tracked in analogy to the way in which the (first)reference device is tracked (cf. the above description relating to thefor example fourth exemplary step of the disclosed method), for examplebecause it is provided with a marker device as described above. Then,the relative position between the first and second marker device can bedetected and used as a basis for determining the position of theanatomical body part in the tracking reference system.

In one example, determining the body part tracking data thereforecomprises:

-   -   acquiring, at the at least one processor, second reference        device tracking data describing a position of the second        reference device in the tracking reference system, wherein a        relative position between the second reference device and the        anatomical body part is predetermined (e.g. at least one of        known or fixed)—the second device tracking data is generated for        example at the point in time at which the patient image data is        generated, and the relative position between the second        reference device and the anatomical body part is then kept in        the predetermined relationship (for example, kept fixed);    -   determining, based on the reference device tracking data and the        second reference device tracking data, reference device relative        position data describing a relative position between the        reference device (i.e. the first reference device) and the        second reference device (e.g. by establishing a linear mapping        and/or a positional difference vector between the positions of        the first and second reference device which have been detected        by the tracking in the tracking reference system);    -   determining the body part tracking data based on the reference        device relative position data, for example by establishing        (calculating) a transformation (i.e. a linear mapping and/or        positional difference which may be embodied by a vector) between        the relative position between the reference devices on the one        hand and the relative position between the second reference        device and the anatomical body part on the other hand.

In a second aspect, the invention is directed to the reference device,which is usable in a navigated medical procedure.

In examples, the reference device has at least one of the following theproperties:

-   -   it is formed in a uniquely orientable manner, for example is        provided with an orientation feature which is visible in for        example a tomographic image of the reference device (for        example, the orientation feature may be recess or a cut having a        specific direction and being formed on an outer surface of the        reference device which constitutes an easily recognizable image        feature in a tomographic image such a CT image of the reference        device);    -   at least part of it is opaque for x-rays (in this case, the        reference device, for example a base part of the reference        device, may comprise a high-density plastic material such as        PEEK—polyether etherketone or PET—polyethylene terephthalate) or        magnetic resonance imaging (in this case, the reference device        may comprise a fat-water-marker which is visible in a magnetic        resonance image of the reference device);    -   it includes a (for example closed) cavity containing        gadopentetic acid (for example, Magnevist®);    -   at least three optical or electromagnetic markers are attached        to it in a predetermined (for example at least one of known or        fixed) spatial relationship (for example relative to the        reference device and/or to one another);    -   it is provided with an attachment means (like a recess in a        lateral side of the reference device) for (for example fixedly        or non-fixedly) attaching it to a second reference device        comprising for example a spine clamp;    -   it shows no symmetry at least on its surfaces, i.e. the        reference device is asymmetric, for example entirely asymmetric        at least concerning its surface appearance and/or surface shape        (in this example, the reference device does not display any        symmetry according to any possible two- or three-dimensional        symmetry group);    -   at least one of at least one recess or at least one projection        is provided on an external surface (in one example, at only one        for example predetermined location on the entire outer surface)        of a base part of the reference device (this feature can enhance        the precision of determining the position of the reference        device in the digital image).

In a third aspect, the invention is directed to use of the referencedevice according to the preceding claim in a registration procedure fordetermining a transformation (e.g. a linear mapping) between the imagereference system and the tracking reference system, wherein for examplethe reference device according to the second aspect (the first referencedevice) is positioned relative to a second reference device for examplewithout establishing a structural connection between the two (first andsecond) reference devices.

In a fourth aspect, the invention is directed to a computer programwhich, when running on at least one processor (for example, a processor)of at least one computer (for example, a computer) or when loaded intoat least one memory (for example, a memory) of at least one computer(for example, a computer), causes the at least one computer to performthe above-described method according to the first aspect.

In a fifth aspect, the invention is directed to a non-transitorycomputer-readable program storage medium on which the program accordingto the fourth aspect is stored.

In a sixth aspect, the invention is directed to at least one computer(for example, a computer), comprising at least one processor (forexample, a processor) and at least one memory (for example, a memory),wherein the program according to the fourth aspect is running on theprocessor or is loaded into the memory, or wherein the at least onecomputer comprises the computer-readable program storage mediumaccording to the fifth aspect.

In a seventh aspect, the invention is directed to a system forsupporting determining a transformation defining a geometricrelationship between a position of an anatomical body part of a patientin an image reference system and a position of the anatomical body partin a tracking reference system the system comprising:

-   -   a) the at least one computer according to the sixth aspect;    -   b) at least one electronic data storage device storing at least        the patient image data and the reference device model data; and    -   c) a medical imaging device (for example, a tomographic imaging        device such as an x-ray computer tomograph, a cone beam CT or a        magnetic resonance tomograph, or an ultrasound or an imaging        device usable for PET or EIT or SPEC) for taking the patient        image data, the medical imaging device being operably coupled to        the at least one computer for transmitting a signal to the at        least one computer corresponding to the patient image data,        -   wherein the at least one computer is operably coupled to the            at least one electronic data storage device for acquiring,            from the at least one data storage device, at least one of            the patient image data or the reference device model data.

In an example, the system according to the seventh aspect comprises:

-   -   d) a marker detection device for detecting the position of at        least one optical or electromagnetic marker,        -   wherein the marker detection device is operably coupled to            the at least one computer for transmitting a signal to the            at least one computer corresponding to the reference device            tracking data.

In an eighth aspect, the invention is directed to a system for use in anavigated medical procedure, comprising:

-   -   a) the system according to seventh aspect; and    -   b) the reference device according to second aspect.

In general, the invention does not involve or for example comprise orencompass an invasive step which would represent a substantial physicalinterference with the body requiring professional medical expertise tobe carried out and entailing a substantial health risk even when carriedout with the required professional care and expertise. For example, theinvention does not comprise a step of irradiating the anatomical bodypart and/or the patient's body with ionizing radiation so that it doesnot comprise any steps of therapy of the human or animal body, forexample it does not comprise any step of radiotherapy or radiosurgery.More particularly, the invention does not involve or in particularcomprise or encompass any surgical or therapeutic activity. Theinvention is instead directed as applicable to positioning a patientrelative to the treatment device for example before any radiotherapy orradiosurgery ensues. For this reason alone, no surgical or therapeuticactivity and in particular no surgical or therapeutic step isnecessitated or implied by carrying out the invention.

It is within the scope of the present invention to combine one or morefeatures of one or more embodiments or aspects of the invention in orderto form a new embodiment wherever this is technically expedient and/orfeasible. Specifically, a feature of one embodiment which has the sameor a similar function to another feature of another embodiment can beexchanged with said other feature, and a feature of one embodiment whichadds an additional function to another embodiment can for example beadded to said other embodiment.

Definitions

In this section, definitions for specific terminology used in thisdisclosure are offered which also form part of the present disclosure.

The method in accordance with the invention is for example a computerimplemented method. For example, all the steps or merely some of thesteps (i.e. less than the total number of steps) of the method inaccordance with the invention can be executed by a computer (forexample, at least one computer). An embodiment of the computerimplemented method is a use of the computer for performing a dataprocessing method. An embodiment of the computer implemented method is amethod concerning the operation of the computer such that the computeris operated to perform one, more or all steps of the method.

The computer for example comprises at least one processor and forexample at least one memory in order to (technically) process the data,for example electronically and/or optically. The processor being forexample made of a substance or composition which is a semiconductor, forexample at least partly n- and/or p-doped semiconductor, for example atleast one of II-, III-, IV-, V-, VI-semiconductor material, for example(doped) silicon and/or gallium arsenide. The calculating steps describedare for example performed by a computer. Determining steps orcalculating steps are for example steps of determining data within theframework of the technical method, for example within the framework of aprogram. A computer is for example any kind of data processing device,for example electronic data processing device. A computer can be adevice which is generally thought of as such, for example desktop PCs,notebooks, netbooks, etc., but can also be any programmable apparatus,such as for example a mobile phone or an embedded processor. A computercan for example comprise a system (network) of “sub-computers”, whereineach sub-computer represents a computer in its own right. The term“computer” includes a cloud computer, for example a cloud server. Theterm “cloud computer” includes a cloud computer system which for examplecomprises a system of at least one cloud computer and for example aplurality of operatively interconnected cloud computers such as a serverfarm. Such a cloud computer is preferably connected to a wide areanetwork such as the world wide web (WWW) and located in a so-calledcloud of computers which are all connected to the world wide web. Suchan infrastructure is used for “cloud computing”, which describescomputation, software, data access and storage services which do notrequire the end user to know the physical location and/or configurationof the computer delivering a specific service. For example, the term“cloud” is used in this respect as a metaphor for the Internet (worldwide web). For example, the cloud provides computing infrastructure as aservice (IaaS). The cloud computer can function as a virtual host for anoperating system and/or data processing application which is used toexecute the method of the invention. The cloud computer is for examplean elastic compute cloud (EC2) as provided by Amazon Web Services™. Acomputer for example comprises interfaces in order to receive or outputdata and/or perform an analogue-to-digital conversion. The data are forexample data which represent physical properties and/or which aregenerated from technical signals. The technical signals are for examplegenerated by means of (technical) detection devices (such as for exampledevices for detecting marker devices) and/or (technical) analyticaldevices (such as for example devices for performing (medical) imagingmethods), wherein the technical signals are for example electrical oroptical signals. The technical signals for example represent the datareceived or outputted by the computer. The computer is preferablyoperatively coupled to a display device which allows informationoutputted by the computer to be displayed, for example to a user. Oneexample of a display device is an augmented reality device (alsoreferred to as augmented reality glasses) which can be used as “goggles”for navigating. A specific example of such augmented reality glasses isGoogle Glass (a trademark of Google, Inc.). An augmented reality devicecan be used both to input information into the computer by userinteraction and to display information outputted by the computer.Another example of a display device would be a standard computer monitorcomprising for example a liquid crystal display operatively coupled tothe computer for receiving display control data from the computer forgenerating signals used to display image information content on thedisplay device. A specific embodiment of such a computer monitor is adigital lightbox. The monitor may also be the monitor of a portable, forexample handheld, device such as a smart phone or personal digitalassistant or digital media player.

Within the framework of the invention, computer program elements can beembodied by hardware and/or software (this includes firmware, residentsoftware, micro-code, etc.). Within the framework of the invention,computer program elements can take the form of a computer programproduct which can be embodied by a computer-usable, for examplecomputer-readable data storage medium comprising computer-usable, forexample computer-readable program instructions, “code” or a “computerprogram” embodied in said data storage medium for use on or inconnection with the instruction-executing system. Such a system can be acomputer; a computer can be a data processing device comprising meansfor executing the computer program elements and/or the program inaccordance with the invention, for example a data processing devicecomprising a digital processor (central processing unit or CPU) whichexecutes the computer program elements, and optionally a volatile memory(for example a random access memory or RAM) for storing data used forand/or produced by executing the computer program elements. Within theframework of the present invention, a computer-usable, for examplecomputer-readable data storage medium can be any data storage mediumwhich can include, store, communicate, propagate or transport theprogram for use on or in connection with the instruction-executingsystem, apparatus or device. The computer-usable, for examplecomputer-readable data storage medium can for example be, but is notlimited to, an electronic, magnetic, optical, electromagnetic, infraredor semiconductor system, apparatus or device or a medium of propagationsuch as for example the Internet. The computer-usable orcomputer-readable data storage medium could even for example be paper oranother suitable medium onto which the program is printed, since theprogram could be electronically captured, for example by opticallyscanning the paper or other suitable medium, and then compiled,interpreted or otherwise processed in a suitable manner. The datastorage medium is preferably a non-volatile data storage medium. Thecomputer program product and any software and/or hardware described hereform the various means for performing the functions of the invention inthe example embodiments. The computer and/or data processing device canfor example include a guidance information device which includes meansfor outputting guidance information. The guidance information can beoutputted, for example to a user, visually by a visual indicating means(for example, a monitor and/or a lamp) and/or acoustically by anacoustic indicating means (for example, a loudspeaker and/or a digitalspeech output device) and/or tactilely by a tactile indicating means(for example, a vibrating element or a vibration element incorporatedinto an instrument). For the purpose of this document, a computer is atechnical computer which for example comprises technical, for exampletangible components, for example mechanical and/or electroniccomponents. Any device mentioned as such in this document is a technicaland for example tangible device.

The expression “acquiring data” for example encompasses (within theframework of a computer implemented method) the scenario in which thedata are determined by the computer implemented method or program.Determining data for example encompasses measuring physical quantitiesand transforming the measured values into data, for example digitaldata, and/or computing the data by means of a computer and for examplewithin the framework of the method in accordance with the invention. Themeaning of “acquiring data” also for example encompasses the scenario inwhich the data are received or retrieved by the computer implementedmethod or program, for example from another program, a previous methodstep or a data storage medium, for example for further processing by thecomputer implemented method or program. Generation of the data to beacquired may but need not be part of the method in accordance with theinvention. The expression “acquiring data” can therefore also forexample mean waiting to receive data and/or receiving the data. Thereceived data can for example be inputted via an interface.

The expression “acquiring data” can also mean that the computerimplemented method or program performs steps in order to (actively)receive or retrieve the data from a data source, for instance a datastorage medium (such as for example a ROM, RAM, database, hard drive,etc.), or via the interface (for instance, from another computer or anetwork). The data acquired by the disclosed method or device,respectively, may be acquired from a database located in a data storagedevice which is operably to a computer for data transfer between thedatabase and the computer, for example from the database to thecomputer. The computer acquires the data for use as an input for stepsof determining data. The determined data can be output again to the sameor another database to be stored for later use. The database or databaseused for implementing the disclosed method can be located on networkdata storage device or a network server (for example, a cloud datastorage device or a cloud server) or a local data storage device (suchas a mass storage device operably connected to at least one computerexecuting the disclosed method). The data can be made “ready for use” byperforming an additional step before the acquiring step. In accordancewith this additional step, the data are generated in order to beacquired. The data are for example detected or captured (for example byan analytical device). Alternatively or additionally, the data areinputted in accordance with the additional step, for instance viainterfaces. The data generated can for example be inputted (for instanceinto the computer). In accordance with the additional step (whichprecedes the acquiring step), the data can also be provided byperforming the additional step of storing the data in a data storagemedium (such as for example a ROM, RAM, CD and/or hard drive), such thatthey are ready for use within the framework of the method or program inaccordance with the invention. The step of “acquiring data” cantherefore also involve commanding a device to obtain and/or provide thedata to be acquired. In particular, the acquiring step does not involvean invasive step which would represent a substantial physicalinterference with the body, requiring professional medical expertise tobe carried out and entailing a substantial health risk even when carriedout with the required professional care and expertise. In particular,the step of acquiring data, for example determining data, does notinvolve a surgical step and in particular does not involve a step oftreating a human or animal body using surgery or therapy. In order todistinguish the different data used by the present method, the data aredenoted (i.e. referred to) as “XY data” and the like and are defined interms of the information which they describe, which is then preferablyreferred to as “XY information” and the like.

In the field of medicine, imaging methods (also called imagingmodalities and/or medical imaging modalities) are used to generate imagedata (for example, two-dimensional or three-dimensional image data) ofanatomical structures (such as soft tissues, bones, organs, etc.) of thehuman body. The term “medical imaging methods” is understood to mean(advantageously apparatus-based) imaging methods (for example so-calledmedical imaging modalities and/or radiological imaging methods) such asfor instance computed tomography (CT) and cone beam computed tomography(CBCT, such as volumetric CBCT), x-ray tomography, magnetic resonancetomography (MRT or MRI), conventional x-ray, sonography and/orultrasound examinations, and positron emission tomography. For example,the medical imaging methods are performed by the analytical devices.Examples for medical imaging modalities applied by medical imagingmethods are: X-ray radiography, magnetic resonance imaging, medicalultrasonography or ultrasound, endoscopy, elastography, tactile imaging,thermography, medical photography and nuclear medicine functionalimaging techniques as positron emission tomography (PET) andSingle-photon emission computed tomography (SPECT), as mentioned byWikipedia. The image data thus generated is also termed “medical imagingdata”. Analytical devices for example are used to generate the imagedata in apparatus-based imaging methods. The imaging methods are forexample used for medical diagnostics, to analyse the anatomical body inorder to generate images which are described by the image data. Theimaging methods are also for example used to detect pathological changesin the human body. However, some of the changes in the anatomicalstructure, such as the pathological changes in the structures (tissue),may not be detectable and for example may not be visible in the imagesgenerated by the imaging methods. A tumour represents an example of achange in an anatomical structure. If the tumour grows, it may then besaid to represent an expanded anatomical structure. This expandedanatomical structure may not be detectable; for example, only a part ofthe expanded anatomical structure may be detectable. Primary/high-gradebrain tumours are for example usually visible on MRI scans when contrastagents are used to infiltrate the tumour. MRI scans represent an exampleof an imaging method. In the case of MRI scans of such brain tumours,the signal enhancement in the MRI images (due to the contrast agentsinfiltrating the tumour) is considered to represent the solid tumourmass. Thus, the tumour is detectable and for example discernible in theimage generated by the imaging method. In addition to these tumours,referred to as “enhancing” tumours, it is thought that approximately 10%of brain tumours are not discernible on a scan and are for example notvisible to a user looking at the images generated by the imaging method.

Image fusion can be elastic image fusion or rigid image fusion. In thecase of rigid image fusion, the relative position between the pixels ofa 2D image and/or voxels of a 3D image is fixed, while in the case ofelastic image fusion, the relative positions are allowed to change.

In this application, the term “image morphing” is also used as analternative to the term “elastic image fusion”, but with the samemeaning.

Elastic fusion transformations (for example, elastic image fusiontransformations) are for example designed to enable a seamlesstransition from one dataset (for example a first dataset such as forexample a first image) to another dataset (for example a second datasetsuch as for example a second image). The transformation is for exampledesigned such that one of the first and second datasets (images) isdeformed, for example in such a way that corresponding structures (forexample, corresponding image elements) are arranged at the same positionas in the other of the first and second images. The deformed(transformed) image which is transformed from one of the first andsecond images is for example as similar as possible to the other of thefirst and second images. Preferably, (numerical) optimisation algorithmsare applied in order to find the transformation which results in anoptimum degree of similarity. The degree of similarity is preferablymeasured by way of a measure of similarity (also referred to in thefollowing as a “similarity measure”). The parameters of the optimisationalgorithm are for example vectors of a deformation field. These vectorsare determined by the optimisation algorithm in such a way as to resultin an optimum degree of similarity. Thus, the optimum degree ofsimilarity represents a condition, for example a constraint, for theoptimisation algorithm. The bases of the vectors lie for example atvoxel positions of one of the first and second images which is to betransformed, and the tips of the vectors lie at the corresponding voxelpositions in the transformed image. A plurality of these vectors ispreferably provided, for instance more than twenty or a hundred or athousand or ten thousand, etc. Preferably, there are (other) constraintson the transformation (deformation), for example in order to avoidpathological deformations (for instance, all the voxels being shifted tothe same position by the transformation). These constraints include forexample the constraint that the transformation is regular, which forexample means that a Jacobian determinant calculated from a matrix ofthe deformation field (for example, the vector field) is larger thanzero, and also the constraint that the transformed (deformed) image isnot self-intersecting and for example that the transformed (deformed)image does not comprise faults and/or ruptures. The constraints includefor example the constraint that if a regular grid is transformedsimultaneously with the image and in a corresponding manner, the grid isnot allowed to interfold at any of its locations. The optimising problemis for example solved iteratively, for example by means of anoptimisation algorithm which is for example a first-order optimisationalgorithm, such as a gradient descent algorithm. Other examples ofoptimisation algorithms include optimisation algorithms which do not usederivations, such as the downhill simplex algorithm, or algorithms whichuse higher-order derivatives such as Newton-like algorithms. Theoptimisation algorithm preferably performs a local optimisation. Ifthere is a plurality of local optima, global algorithms such assimulated annealing or generic algorithms can be used. In the case oflinear optimisation problems, the simplex method can for instance beused.

In the steps of the optimisation algorithms, the voxels are for exampleshifted by a magnitude in a direction such that the degree of similarityis increased. This magnitude is preferably less than a predefined limit,for instance less than one tenth or one hundredth or one thousandth ofthe diameter of the image, and for example about equal to or less thanthe distance between neighbouring voxels. Large deformations can beimplemented, for example due to a high number of (iteration) steps.

The determined elastic fusion transformation can for example be used todetermine a degree of similarity (or similarity measure, see above)between the first and second datasets (first and second images). To thisend, the deviation between the elastic fusion transformation and anidentity transformation is determined. The degree of deviation can forinstance be calculated by determining the difference between thedeterminant of the elastic fusion transformation and the identitytransformation. The higher the deviation, the lower the similarity,hence the degree of deviation can be used to determine a measure ofsimilarity.

A measure of similarity can for example be determined on the basis of adetermined correlation between the first and second datasets.

It is the function of a marker to be detected by a marker detectiondevice (for example, a camera or an ultrasound receiver or analyticaldevices such as CT or MRI devices) in such a way that its spatialposition (i.e. its spatial location and/or alignment) can beascertained. The detection device is for example part of a navigationsystem. The markers can be active markers. An active marker can forexample emit electromagnetic radiation and/or waves which can be in theinfrared, visible and/or ultraviolet spectral range. A marker can alsohowever be passive, i.e. can for example reflect electromagneticradiation in the infrared, visible and/or ultraviolet spectral range orcan block x-ray radiation. To this end, the marker can be provided witha surface which has corresponding reflective properties or can be madeof metal in order to block the x-ray radiation. It is also possible fora marker to reflect and/or emit electromagnetic radiation and/or wavesin the radio frequency range or at ultrasound wavelengths. A markerpreferably has a spherical and/or spheroid shape and can therefore bereferred to as a marker sphere; markers can however also exhibit acornered, for example cubic, shape.

A marker device can for example be a reference star or a pointer or asingle marker or a plurality of (individual) markers which are thenpreferably in a predetermined spatial relationship. A marker devicecomprises one, two, three or more markers, wherein two or more suchmarkers are in a predetermined spatial relationship. This predeterminedspatial relationship is for example known to a navigation system and isfor example stored in a computer of the navigation system.

In another embodiment, a marker device comprises an optical pattern, forexample on a two-dimensional surface. The optical pattern might comprisea plurality of geometric shapes like circles, rectangles and/ortriangles. The optical pattern can be identified in an image captured bya camera, and the position of the marker device relative to the cameracan be determined from the size of the pattern in the image, theorientation of the pattern in the image and the distortion of thepattern in the image. This allows to determine the relative position inup to three rotational dimensions and up to three translationaldimensions from a single two-dimensional image.

The position of a marker device can be ascertained, for example by amedical navigation system. If the marker device is attached to anobject, such as a bone or a medical instrument, the position of theobject can be determined from the position of the marker device and therelative position between the marker device and the object. Determiningthis relative position is also referred to as registering the markerdevice and the object. The marker device or the object can be tracked,which means that the position of the marker device or the object isascertained twice or more over time.

A pointer is a rod which comprises one or more—advantageously,two—markers fastened to it and which can be used to measure offindividual co-ordinates, for example spatial co-ordinates (i.e.three-dimensional co-ordinates), on a part of the body, wherein a userguides the pointer (for example, a part of the pointer which has adefined and advantageously fixed position with respect to the at leastone marker attached to the pointer) to the position corresponding to theco-ordinates, such that the position of the pointer can be determined byusing a surgical navigation system to detect the marker on the pointer.The relative location between the markers of the pointer and the part ofthe pointer used to measure off co-ordinates (for example, the tip ofthe pointer) is for example known. The surgical navigation system thenenables the location (of the three-dimensional co-ordinates) to beassigned to a predetermined body structure, wherein the assignment canbe made automatically or by user intervention.

A “reference star” refers to a device with a number of markers,advantageously three markers, attached to it, wherein the markers are(for example detachably) attached to the reference star such that theyare stationary, thus providing a known (and advantageously fixed)position of the markers relative to each other. The position of themarkers relative to each other can be individually different for eachreference star used within the framework of a surgical navigationmethod, in order to enable a surgical navigation system to identify thecorresponding reference star on the basis of the position of its markersrelative to each other. It is therefore also then possible for theobjects (for example, instruments and/or parts of a body) to which thereference star is attached to be identified and/or differentiatedaccordingly. In a surgical navigation method, the reference star servesto attach a plurality of markers to an object (for example, a bone or amedical instrument) in order to be able to detect the position of theobject (i.e. its spatial location and/or alignment). Such a referencestar for example features a way of being attached to the object (forexample, a clamp and/or a thread) and/or a holding element which ensuresa distance between the markers and the object (for example in order toassist the visibility of the markers to a marker detection device)and/or marker holders which are mechanically connected to the holdingelement and which the markers can be attached to.

The present invention is also directed to a navigation system forcomputer-assisted surgery. This navigation system preferably comprisesthe aforementioned computer for processing the data provided inaccordance with the computer implemented method as described in any oneof the embodiments described herein. The navigation system preferablycomprises a detection device for detecting the position of detectionpoints which represent the main points and auxiliary points, in order togenerate detection signals and to supply the generated detection signalsto the computer, such that the computer can determine the absolute mainpoint data and absolute auxiliary point data on the basis of thedetection signals received. A detection point is for example a point onthe surface of the anatomical structure which is detected, for exampleby a pointer. In this way, the absolute point data can be provided tothe computer. The navigation system also preferably comprises a userinterface for receiving the calculation results from the computer (forexample, the position of the main plane, the position of the auxiliaryplane and/or the position of the standard plane). The user interfaceprovides the received data to the user as information. Examples of auser interface include a display device such as a monitor, or aloudspeaker. The user interface can use any kind of indication signal(for example a visual signal, an audio signal and/or a vibrationsignal). One example of a display device is an augmented reality device(also referred to as augmented reality glasses) which can be used asso-called “goggles” for navigating. A specific example of such augmentedreality glasses is Google Glass (a trademark of Google, Inc.). Anaugmented reality device can be used both to input information into thecomputer of the navigation system by user interaction and to displayinformation outputted by the computer.

DESCRIPTION OF THE FIGURES

In the following, the invention is described with reference to theappended figures which represent a specific embodiment of the invention.The scope of the invention is however not limited to the specificfeatures disclosed in the context of the figures, wherein

FIG. 1 is a flow diagram showing the basic steps of the disclosed methodaccording to the first aspect;

FIG. 2 is a view of the reference device according to the second aspectfrom an oblique angle;

FIG. 3 is a view of the reference device according to the second aspectin a +z-direction;

FIG. 4 is a view of the reference device according to the second aspectin a +x-direction direction;

FIG. 5 is a view of the reference device according to the second aspectin a −x-direction;

FIG. 6 is a view of the reference device according to the second aspectin a −y-direction;

FIG. 7 is a view of the reference device according to the second aspectin a +y-direction;

FIG. 8 is a view of a second reference device comprising a spine clamp;

FIG. 9 is an illustration of the reference device according to thesecond aspect in combination with a second reference; and

FIG. 10 is a view of setup for treating a patient using the combinationof FIG. 9 on a patient in combination with a navigation system.

FIG. 1 is a flow diagram illustrating the basic steps of the disclosedmethod in accordance with the first aspect, which in the illustrativeexample of FIG. 1 starts with a step S11 of acquiring the patient imagedata. In subsequent step S12, the reference device model data isacquired, followed by step S13 which encompasses determining thereference device image position data. Then, step S14 acquires thereference device tracking data. Steps S11 to S14 serve as input stepsfor the last step shown in FIG. 1 which is step S15 encompassingdetermination of the body part tracking data.

A specific example of using the setup of FIG. 10 or a setup beingtechnically equivalent to the setup of FIG. 10 comprises the followingmethod steps, which fall into the scope of the method in accordance withthe first aspect:

1. The tracking markers of the first reference device are registered tothe second reference device (also called dynamic reference) before orafter taking the scan for generating the patient image data (resultingin Transformation 1, also abbreviated as T1, the information is includedin the reference device tracking data). The dynamic reference defines aphysical space for image-guidance of instruments, robots or otherdevices.2. Scanning for generation of the patient image data.3. The data resulting from the scanning (the patient image data) istransferred to the computer of the navigation system.4. The patient image data is searched (e.g. by the computer of thenavigation system) for the image representation of the (first) referencedevice (also called registration device) and a surface match isperformed between known geometric data of the device (the referencedevice model data) and surface data (describing specifically theexternal surface) of the first reference device extracted the patientimage data. Thereby, the reference device image position data isdetermined. A transformation from image space (the image referencesystem) to geometric space of the registration device (the trackingreference system) is performed (resulting in Transformation 2, alsoabbreviated as T2).5. The geometry of the first reference device is known from thereference device model data, therefore the transformation from thesurface of the first reference device to the tracking markers 12 of thesecond reference device is known in geometric space of the registrationdevice, i.e. in the tracking reference system (resulting inTransformation 3, also abbreviated as T3).6. T2+T3 is the transformation from image space (“the scan”), from theimage reference system, to the tracking markers of the first referencedevice, i.e. to the tracking reference system. The transformation fromthe tracking markers to the second reference device is known as well (asT1), therefore the image reference system can now be registered totracking reference system e.g. by the software (the correspondingtransformation being represented as T1+T2+T3). T1+T2+T3 allows todetermine the body part tracking data.

Variants of the sequence are:

First Variant:

1. The position of the first reference device is registered to theposition of the second reference device before or after the scan(resulting in Transformation 1, also abbreviated as T1). The positionsare defined by the positions of the respective tracking markers and aredetermined in this example by optical tracking.2. Scanning, i.e. generating the patient image data.3. The data resulting from the scanning (the patient image data) istransferred to the computer of the navigation system.4. The geometry of the registration device is known from the firstreference device model data, therefore the transformation from thesurface of the first reference device to the tracking markers of thesecond reference device is known in the tracking reference system(resulting in Transformation 3, also abbreviated as T3).5. The software searches the patient image data for the representationof the first reference device and performs a surface match between themodel of the first reference device and surface data of the devicedetermined in the patient image data. A transformation from the imagereference system to the tracking reference system is performed(Transformation 2). Thereby, the reference device image position data isdetermined.6. T2+T3 is the transformation from the image reference system to thetracking markers of the first reference device, i.e. the trackingreference system. The transformation from the tracking markers to thesecond reference device is known as well (as T, the information isincluded in the reference device tracking data 1), therefore the imagereference system can now be registered to the tracking reference systeme.g. by the software (the corresponding transformation being representedas T1+T2+T3). T1+T2+T3 allows to determine the body part tracking data.Second Variant:1. Scanning, i.e. generating the patient image data.2. The data resulting from the scanning (the patient image data) istransferred to the computer of the navigation system.3. The software searches the patient image data for the representationof the first reference device and performs a surface match between themodel of the first reference device and surface data of the devicedetermined in the patient image data. A transformation from the imagereference system to the tracking reference system is performed(resulting in Transformation 2, also abbreviated as T2). Thereby, thereference device image position data is determined.4. The geometry of the registration device is known from the firstreference device model data, therefore the transformation from thesurface of the first reference device to the tracking markers of thesecond reference device is known in the tracking reference system(resulting in Transformation 3, also abbreviated as T3).5. The position of the first reference device is registered to theposition of the second reference device before or after the scan of step1 (resulting in Transformation 1, also abbreviated as T1, theinformation is included in the reference device tracking data). Thepositions are defined by the positions of the respective trackingmarkers and are determined in this example by optical tracking.6. T2+T3 is the transformation from the image reference system to thetracking reference system. The transformation from the tracking markersto the dynamic reference is known as well (as T1), therefore the imagespace can now be registered to physical space by the software (thecorresponding transformation being represented as T1+T2+T3). T1+T2+T3allows to determine the body part tracking data.

This registration may now be used for image-based guidance of aninstrument, a robot or another for example medical device (e.g.microscope such as a microscope usable in a surgical procedure).

In another embodiment, it is also possible to not use the secondreference device. Instead, the first reference device needs to have aknown, for example fixed, specifically a rigid, connection to thepatient. The instrument, robot or other device to be image-guided needsto have a known, for example fixed, specifically a rigid, connection tothe registration device or to the patient as well. This embodiment thenproceeds as follows:

1. Scanning, i.e. generating the patient image data.

2. The data resulting from the scanning (the patient image data) istransferred to the computer of the navigation system.

3. The patient image data is searched (e.g. by the computer of thenavigation system) for the image representation of the (first) referencedevice (also called registration device) and a surface match isperformed between known geometric data of the device (the referencedevice model data) and surface data (describing specifically theexternal surface) of the first reference device extracted the patientimage data. Thereby, the reference device image position data isdetermined. A transformation from image space (the image referencesystem) to geometric space of the registration device (the trackingreference system) is The geometry of the registration device is knownfrom the first reference device model data, therefore the transformationfrom the surface of the first reference device to the tracking markersof the second reference device is known in the tracking reference system(resulting in Transformation 3, also abbreviated as T3).5. T2+T3 is the transformation from the image reference system to thetracking markers of the first reference device, i.e. the trackingreference system. The transformation from the tracking markers to thesecond reference device is known as well (as T1, the information isincluded in the reference device tracking data), therefore the imagereference system can now be registered to the tracking reference systeme.g. by the software (the corresponding transformation being representedas T1+T2+T3) for tracking of the instrument, robot or other device inthe tracking reference system. T1+T2+T3 also allows to determine thebody part tracking data.

The registration can now be used to guide the instrument, robot or otherdevice directly without the need for a dynamic reference.

The above-mentioned surface match comprises the following steps: Thesurface geometry of the first reference device is known from thereference device model data. A number of points on the surface of theregistration device is defined. In the patient image data, a thresholdis defined (or calculated) that defines the surface of therepresentation of the registration device in the patient image data. Acomplete search over the whole volume, i.e. the whole of the patientimage data is performed until a perfect match of the known surfacepoints with the corresponding surface geometry in the patient image datais found. The search over the whole volume can be reduced if thelocation of the image representation of the first reference device isapproximately known before, or by using a rough search before the actualmatch. With this match, Transformation 2 is determined: thetransformation from the image space (image reference system) to thegeometric space of the registration device (the tracking referencesystem).

As an alternative to the surface match described above, an fusion matchcan be used, which comprises the following steps: From the geometry ofthe first reference device known from the reference device model data, asynthetic image data set is computed whose coordinate space has a knownrelation to the geometric space of the registration device (the trackingreference system). This synthetic image data set is fused with thepatient image data using suitable similarity measures such ascorrelations or mutual information. This is done by translating androtating one data set over the other and calculating the similaritymeasure for each pose. The pose with the highest similarity is thewinning fusion match, i.e. the location of the registration device inthe scan data. With this match, Transformation 2 is determined: thetransformation from the image space (image reference system) to thegeometric space of the registration device (the tracking referencesystem).

FIG. 2 shows a view from an oblique perspective onto the first)reference device 1. The first reference device 1 is composed of ablock-like base part 2 having handles (for example two handles) 3 a, 3b. The base part 2 has rounded corners in order to avoid injury to thepatient e.g. when the block is placed near the anatomical body part. Thebase part is provided with four markers devices embodied by markers 4(in the example shown in the figures, on the upper side of the base part2 which faces the +y-direction of the coordinate system shown in FIGS. 2to 7), in this case optical markers having a reflective sphere 5 and anattachment part 6 for connection of the reflective sphere 5 with thebase part 2. The base part 2 is also provided with four feet 7 forstable placement of the base part onto a surface such as the patient'sback. The feet 7 are provided on a lower side of the base part 2 whichface the −y-direction of the coordinate system shown in FIGS. 2 to 7).

FIGS. 2 to 7 show different views of the first reference device 1,wherein the same reference signs denote the same features.

As can be seen from FIGS. 2 to 8, reference device 1 has numerousgeometric features (also called orientation features) which generateasymmetry of the first reference device 1 in all possible directions:

-   -   the lateral surfaces of the base part 2 facing the +x/−x-,        +y/−y-, +z/−z-directions have no portion which is parallel to an        opposing portion of an opposing lateral surface (i.e. of a        lateral surface facing in the x-, y- or z-direction,        respectively, having the opposite sign of the direction);    -   the exterior surface of the base part 2 are provided with an        irregular pattern of recesses 8 a, 8 b, 8 c, 8 d, 8 e, 8 f, 8 g,        8 h, 8 i, 8 k, 81, 8 m and 8 n which displays no two- or        three-dimensional symmetry: first of all, the recesses comprise        recesses having different geometries (e.g. the circular or        cylinder-shaped recesses 8 d, 8 e, 8 f, 8 m and 8 n) and the        cuboid-shaped recesses 8 a, 8 b and 8 c);    -   the markers 4 are provided on the upper surface in a positional        pattern which does not display any two-dimensional (planar)        symmetry (in the plane in which they (i.e. at least the marker        spheres 5) are positioned;    -   the base part 2 should is asymmetric in relation to each of its        main axes (i.e. displays no rotational or translational symmetry        along any of those axes).

The above of features of the reference device 1 allow to uniquelydetermine the orientation of the first reference device 1 in a medicalimage (e.g. in the digital image described by the patient image data)containing an image representation of the first reference device 1.Having a plurality of the recesses 8 a to 8 n allows for easier and amore reliable surface matching.

FIG. 8 is an illustration of the second reference device 9 having aclamp 10 (e.g. a spine clamp or a clamp suitable for placement on theexterior of the patient's body without necessitating any surgicalactivity) and a reference star 11. The reference star is provided on theclamp 10 with an adjustable joint so that the relative position betweenthe clamp 10 and the reference star 11 can be adjusted. The referencestar is provided with four reflective marker devices 12 each having anattachment part 14 and a reflective sphere 13. The marker devices aresuitable for optical tracking.

As shown in FIG. 9, the clamp 10 can be attached to the anatomical bodypart, for example a patient's vertebra or be clipped onto the exteriorof a patient's arm. The first reference device 1 can then be placed nearthe clamp 10 (in one example, without being fixed to the secondreference device 9) to allow for an orientable image representation ofthe first reference device 1 having a trackable position relative to thesecond reference device 9 and to thereby allow for establishing atransformation between the image reference system and the trackingreference system. The first reference device 1 and the second referencedevice 9 then form a system 10 of reference devices.

As shown in FIG. 10, the system 10 of reference devices (specifically,the marker devices 4 and 12) can be tracked during a medical procedureduring which the clamp 10 is attached to an anatomical body partbelonging to the patient's body 16. The patient image data is generatedusing an imaging apparatus like a computed x-ray tomograph 19 forimaging the first reference device 1 simultaneously to imaging theanatomical body part so that the digital image described by the patientimage data shows both the first reference device 1 and the anatomicalbody part. The marker devices 4 and 12 are tracked using a navigationsystem having a marker detection device such as a tracking unit (such asan optical tracking unit, embodied in the example of FIG. 10 by astereoscopic camera 20 having lenses 20 a and 20 b). The tracking unitis operatively coupled (via a wired or wireless data link 23) to a dataprocessing unit 22 (a computer) having a digital processor and a memoryand a visual output unit 21 (a display). The tracking unit 20, the datalink 23, the computer 22 and the visual output unit 21 for a navigationsystem. After registration and scanning, the first reference device 1can be removed from the setup. The patient is then tracked by using onlythe second reference 9 which has a rigid connection to the patient'sbody 16 and therefore a fixed position relative to the anatomical bodypart.

Technical advantages of the disclosed method in accordance with thefirst aspect and the disclosed reference device in accordance with thesecond aspect can be outlined as follows:

Using the performance of today's computers, it is possible to registerthe entire device instead of single fiducials (markers). In formertimes, fiducials were necessary to speed up calculation time of theregistration process. Registering only a few single fiducials (3-10)provided good accuracy at an acceptable calculation time. Today, manypoints on the surface of the entire device (200-2000) can be used whilethe calculation time is still acceptable.

Further on, avoiding image-visible, discrete markers has the advantageof better manufacturability of the device. It also avoids deteriorationof image quality in the scan because no materials of higher density arerequired. Eliminating the device from the scan after registrationimproves usability of the scan data during surgery.

Further on, the inner surfaces of a hollow registration device may beused. With today's manufacturing capabilities (3D-printers), a hollowdevice with an irregular shape inside can be easily manufactured so thatthe device might have good properties for cleaning and sterilization butat the same time provide the irregular shape required for a surfacematch or fusion.

For a fusion match also a registration device that has various 3D areasinside having different densities can be used. The shapes inside as wellas the complete shape of the registration device are used for finding afusion match.

The disclosed method may be summarized as follows:

The outer surface of the first reference device is determined in thescan, a surface match is performed with a model of the first referencedevice. and image space is registered to physical space. By using thesurface of the first reference device, the need for image-visible,discrete markers is avoided. The geometry of the first reference deviceneeds to be of irregular shape, so that as many cross-sections aspossible are unique in the image representation of the first referencedevice in the scan. This way, positional mapping is facilitated.

A second method comprises determining the outer and/or inner shape ofthe first reference device in the scan, performing a fusion match with amodel of the first reference device and registering image space tophysical space. By using a fusion match, the need for determining thesurface of the device in the scan is avoided.

A sequence of the two above-mentioned methods above may also be used,i.e. first the surface match is performed and then the fusion match isperformed for fine adjustment, or vice versa. Only one of theabove-mentioned methods is applied.

After registration, the image representation of the first referencedevice is removed from the scan so that the scan shows only anatomicalinformation.

The invention claimed is:
 1. A method for determining a position of animaged anatomical body part of a patient in a tracking reference system,the method comprising executing, on at least one processor of at leastone computer, steps of: a) acquiring, at the at least one processor,patient image data describing a digital image of at least part of areference device and the anatomical body part; b) acquiring, at the atleast one processor, reference device model data describing a model ofat least one of at least one internal surface or at least one externalsurface of the reference device; c) determining, by the at least oneprocessor and based on the patient image data and the reference devicemodel data, reference device image position data describing a relativeposition between the reference device and the anatomical body part; d)acquiring, at the at least one processor, reference device tracking datadescribing a position of the reference device in the tracking referencesystem; e) acquiring, at the at least one processor, second referencedevice tracking data describing a position of a second reference devicein the tracking reference system, wherein a relative position betweenthe second reference device and the anatomical body part ispredetermined; f) determining, by the at least one processor and basedon the reference device tracking data and the second reference devicetracking data, reference device relative position data describing arelative position between the reference device and the second referencedevice; and g) determining, by the at least one processor and based onthe reference device image position data and the reference devicetracking data and the reference device relative position data, body parttracking data describing a position of the anatomical body part in thetracking reference system.
 2. The method according to claim 1, whereinthe relative position between the reference device and the anatomicalbody part is defined in an image reference system.
 3. The methodaccording to claim 2, wherein the body part tracking data is determinedbased on determining a transformation between the position of thereference device in the image reference system and the position of thereference device in the tracking reference system.
 4. The methodaccording to claim 1, wherein the model defines an image appearance ofthe reference device.
 5. The method according to claim 1, wherein themodel has been generated based on digital model image data describing atleast one image of the reference device.
 6. The method according toclaim 5, wherein the digital model image data is tomographic image data.7. The method according to claim 1, wherein the patient image data andthe reference device model data are three-dimensional or two-dimensionaldata.
 8. The method according to claim 1, wherein the patient image datais tomographic image data.
 9. The method according to claim 1, whereinthe model describes a three-dimensional structure of the referencedevice.
 10. The method according to claim 9, wherein the model describesat least one of an internal or a surface structure of the referencedevice.
 11. The method according to claim 10, wherein the internal orsurface structure of the reference device is described by a geometricalgrid.
 12. The method according to claim 1, wherein the reference deviceimage position data is determined by comparing the model to the digitalimage appearance of the reference device in the digital image describedby the patient image data.
 13. The method according to claim 1, whereinthe reference device image position data is determined by applying atleast one of an image fusion algorithm or a surface match algorithm tothe patient image data and the reference device model data.
 14. Themethod according to claim 1, wherein the reference device tracking datahas been generated by tracking at least one optical marker orelectromagnetic marker having a predetermined spatial relationshiprelative to the reference device.
 15. The method according to claim 14,wherein the at least one optical marker or electromagnetic marker isattached to the reference device.
 16. The method according to claim 1,wherein the reference device has at least one of the followingproperties: the reference device is provided with an orientation featurewhich is visible in a tomographic image of the reference device; atleast part of the reference device is opaque for x-rays or magneticresonance imaging; the reference device includes a cavity containinggadopentetic acid; at least three optical or electromagnetic markers areattached to the reference device in a predetermined spatialrelationship; the reference device is provided with an attachment meansfor attaching the reference device to a second reference device; atleast one of at least one recess or at least one projection is providedon an external surface of a base part of the reference device.
 17. Anon-transitory computer-readable storage medium having stored thereoncomputer-executable instructions that, when executed, configure aprocessor to: a) acquire, at the at least one processor, patient imagedata describing a digital image of at least part of a reference deviceand the anatomical body part; b) acquire, at the at least one processor,reference device model data describing a model of at least one of atleast one internal surface or at least one external surface of thereference device; c) determine, by the at least one processor and basedon the patient image data and the reference device model data, referencedevice image position data describing a relative position between thereference device and the anatomical body part; d) acquire, at the atleast one processor, reference device tracking data describing aposition of the reference device in the tracking reference system; e)acquire, at the at least one processor, second reference device trackingdata describing a position of a second reference device in the trackingreference system, wherein a relative position between the secondreference device and the anatomical body part is predetermined acquire,at the at least one processor, second reference device tracking datadescribing a position of a second reference device in the trackingreference system, wherein a relative position between the secondreference device and the anatomical body part is predetermined; f)determine, by the at least one processor and based on the referencedevice tracking data and the second reference device tracking data,reference device relative position data describing a relative positionbetween the reference device and the second reference device; and g)determine, by the at least one processor and based on the referencedevice image position data and the reference device tracking data andthe reference device relative position data, body part tracking datadescribing a position of the anatomical body part in the trackingreference system.
 18. A system for supporting determining atransformation defining a geometric relationship between a position ofan anatomical body part of a patient in an image reference system and aposition of the anatomical body part in a tracking reference system, thesystem comprising: a) at least one electronic data storage devicestoring at least the patient image data and the reference device modeldata; and b) a medical imaging device for taking the patient image data,the medical imaging device being operably coupled to at least oneprocessor for transmitting a signal to the at least one processorcorresponding to the patient image data, wherein the at least oneprocessor is operably coupled to the at least one electronic datastorage device for acquiring, from the at least one data storage device,at least one of the patient image data or the reference device modeldata, and the at least one processor is further configured to: a)acquire, at the at least one processor, patient image data describing adigital image of at least part of a reference device and the anatomicalbody part; b) acquire, at the at least one processor, reference devicemodel data describing a model of at least one of at least one internalsurface or at least one external surface of the reference device; c)determine, by the at least one processor and based on the patient imagedata and the reference device model data, reference device imageposition data describing a relative position between the referencedevice and the anatomical body part; d) acquire, at the at least oneprocessor, reference device tracking data describing a position of thereference device in the tracking reference system; e) acquire at the atleast one processor, second reference device tracking data describing aposition of a second reference device in the tracking reference system,wherein a relative position between the second reference device and theanatomical body part is predetermined; f) determine, by the at least oneprocessor and based on the reference device tracking data and the secondreference device tracking data, reference device relative position datadescribing a relative position between the reference device and thesecond reference device; and g) determine, by the at least one processorand based on the reference device image position data and the referencedevice tracking data and the reference device relative position data,body part tracking data describing a position of the anatomical bodypart in the tracking reference system.
 19. The system according to claim18, comprising: a marker detection device for detecting the position ofat least one optical or electromagnetic marker, wherein the markerdetection device is operably coupled to the at least one processor fortransmitting a signal to the at least one processor corresponding to thereference device tracking data.